To save everyone from the math, Greased has higher ignition rate. The flamer is unique because it does not have a secondary damage type; the extra ignition chance from incendiary is only applied once. For your own comparison, it's Greased 587% fire chance per second, Incendiary 490% fire chance per second.
I look at Incendiary Rounds as an extension of what the Heavy Carronade can do, like a special function. Just as it is important to look at the Heavy Carronade as a part of a ship - e.g. Blenderfish or Brawling Spire. Simply put, I don't think Incendiary needs to be versatile at all.
@Pies: -30% > -10% RoF will make Incendiary Flamer have significantly higher ignition rate than Greased.